RFR: 8333444: Parallel: Inline PSParallelCompact::mark_obj

[Albert Mingkun Yang]

Trivial inlining a method and removing redundant `is_unmarked` check.

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Commit messages:
 - pgc-inline-mark

Changes: https://git.openjdk.org/jdk/pull/19526/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19526&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333444
  Stats: 22 lines in 3 files changed: 6 ins; 15 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19526.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19526/head:pull/19526

PR: https://git.openjdk.org/jdk/pull/19526